1. Field of the Invention
The invention relates to a twisted pair wire and a twisted pair cable for use in a high-frequency range, such as for a LAN (local area network) or differential signals, etc.
2. Description of the Related Art
It is prohibited to use a stranded conductor for a major part of a LAN cable (90 m of 100 m-link). This is because humidity (moisture) enters into a gap between strands of the stranded conductor (hereinafter, referred to as clearance space) during long-term use, which increases high-frequency resistance and leakage conductance and then increases transmission loss/attenuation (hereinafter, referred to as attenuation).
On the other hand, a stranded conductor is used for a short link such as a working LAN cable for the industrial purpose since flexibility thereof makes laying easy and allows use in vibrating equipments. However, transmission loss increases during long-term use in case of being used for a relatively long distance, which causes failure such as system error or deterioration in data BER (bit error rate).
FIG. 7 is a cross sectional view showing a conventional twisted pair cable using a stranded conductor which is used for a short link.
A twisted pair cable 70 has a stranded conductor 13 formed by twisting seven strands 12 together in a 6-around-1 configuration (a strand 12 located in the middle is surrounded by six helically twisted strands 12). The stranded conductor 13 is covered with a covering body 33 to form a covered strand 72 and a twisted pair wire 71 formed by twisting a pair of the covered strands 72 is further covered with a bundling covering body 16, thereby forming the twisted pair cable 70.
The related art of the invention may be disclosed in JP-A-2001-6452.
In order to manufacture a stranded conductor, it is necessary to twist strands. If strands are not twisted, it is only a bunch of strands and the strands are tangled with each other when wound around a bobbin, hence, it is not possible to manufacture.
On a stranded conductor formed by twisting strands (strand diameter d) at a twist angle A, contact surfaces between strands are present in a length direction of the stranded conductor at a pitch of distance-L=d/cos A as shown in FIG. 8. Therefore, equivalent volume resistivity (high-frequency resistance) as the stranded conductor is not volume resistivity of the conductor per se but resistivity of volume composed of the conductor (major portion), contact surface between the strands and the clearance space therebetween.
Here, relations among attenuation (α), resistance attenuation (αR), leakage attenuation (αg), equivalent volume resistivity (ρ) and dielectric tangent (tan δ) of the stranded conductor are represented by the following formulas (1) to (3), where an equivalent skin thickness on which high-frequency current alternately propagates is t, a diameter of the stranded conductor is D, frequency is f, characteristic impedance of the pair is Zc mutual capacitance of the pair is C and leakage conductance is G=2πfC tan δ.α=αR+αg  (1)αR=2ρ/(πDt)/(2Zc)  (2)αg=GZc/2  (3)
When a stranded conductor is used for a long period of time, moisture enters into a gap between strands and metal is corroded. Accordingly, irregularity is formed surface of the strand due to the corrosion, which results in that the strands are not in surface contact but in point contact with each other. As a result, contact resistance increases by a factor of 1.3 to several times. Furthermore, when the twist angle A is small, the distance L also becomes small and frequency of clearance space in a length direction of the stranded conductor (frequency of contact between strands) is increased, influence of the increase in the contact resistance is thus augmented and the equivalent volume resistivity ρ (high-frequency resistance) is increased more than 10 times. This increases the resistance attenuation of the entire attenuation and becomes a main cause of failure. In addition, when the humidity in the clearance space is increased, tan δ (leakage conductance G) deteriorates since an electric field between conductors also includes the clearance space, which increases leakage attenuation of the entire attenuation and becomes a sub cause of failure.